Electric power steering device

ABSTRACT

A subject of the present invention is to provide an electric power steering capable of improving a contact ratio by using an hourglass worm to attain a higher output, and also performing a correction of a misalignment easily by simplifying remarkably a fitting operation of the hourglass worm.  
     A worm gear system causes a hourglass worm  2  driven by an electric motor  4  to mesh with a worm wheel  3  provided to an output shaft  5 , and a bearing  7  on the shaft end side, which bears rotatably the hourglass worm  2  is composed of a tapered roller bearing an outer ring of which can be separated. In a fitting operation, an inner ring  7   a  and rolling elements  7   b  are fitted into the hourglass worm  2  and also an outer ring  7   c  is fitted in advance in a gear housing  1 . Then, the tapered roller bearing  7  is assembled in the gear housing  1  by moving the hourglass worm  2  obliquely to a rotating shaft line of the hourglass worm  2  in the gear housing  1  along a raceway of the outer ring  7   c.

TECHNICAL FIELD

The present invention relates to an electric power steering forgenerating an assist steering torque from an electric motor in responseto a steering torque applied to a steering wheel, and then transmittingthe assist steering torque to an output shaft of a steering equipmentafter a speed reduction using a worm gear system.

BACKGROUND ART

In the field of the vehicle steering system, the so-called powersteering for giving the steering assist by using the external powersource is widely employed. In the prior art, the hydraulic vane pump isused as the power source of the power steering, and is driven by theengine in most cases. However, it was difficult to employ the powersteering of this type in a subcompact car of a small engine capacity,etc. because the hydraulic pump must be driven all the time and thus aheavy driving loss of the engine (almost several horsepower to severaltens horsepower at the maximum load) is caused. Further, it wasunavoidable that a driving fuel consumption is reduced to anunnegligible extent even in the car of a relatively large enginecapacity.

For this reason, as the power steering that is able to overcome aboveproblems, the electric power steering (abbreviated as an “EPS”hereinafter) using an electric motor as the power source is highlightedrecently. The EPS has several advantages such that the direct drivingloss of the engine can be eliminated because the onboard battery is usedas an electric power supply of the electric motor, a reduction of thedriving fuel consumption can be suppressed because the electric motor isstarted only at the time of steering assist, the electronic control canbe applied extremely readily, and others.

In this EPS, the assist steering torque is generated from the electricmotor in response to the steering torque applied to the steering wheel,and then transmitted to the output shaft of the steering equipment viathe power transmission device (reduction gear).

In the EPS employing the worm gear system as this power transmissiondevice (reduction gear), the worm wheel is engaged with the worm on thedriving shaft side of the electric motor, and then this worm wheel isfitted onto an output shaft (e.g., a pinion shaft or a column shaft) ofthe steering equipment.

By the way, the grease development of the worm reduction gear and theresin material development are executed in grappling with the higheroutput of the EPS. But it is in an awkward situation that theperformance of the EPS should be improved tremendously from an aspect ofthe material. For this reason, the development of the hourglass wormreduction gear that must be able to achieve a breakthrough from anaspect of the mechanism is proceeding in recent years.

The worm reduction gears used up to now are the cylindrical wormreduction gear. The hourglass worm takes an hourglass shape literally incontrast to the cylindrical worm such that a wheel profile is envelopedin the worm. Therefore, it is apparent to everybody that a contact ratio(number) can be improved.

For instance, as the worm gear reduction pair using the cylindricalworm, JP-A-2001-270450 and JP-A-2002-173041 can be listed. In thisevent, as the EPS using the cylindrical worm, as shown in FIG. 45C ofthis application, a cylindrical worm b and a worm wheel c that isengaged with the cylindrical worm b are incorporated into a gear housinga of the warm gear system, and also an electric motor d for driving thecylindrical worm b is mounted on the side of the gear housing a. Theworm wheel c is fitted onto an output shaft e (e.g., the pinion shaft orthe column shaft) of the steering equipment. Accordingly, the assiststeering torque is generated from the electric motor d in response tothe steering torque applied to the steering wheel (not shown), and thentransmitted to the output shaft e of the steering equipment via thecylindrical worm b and the worm wheel c as the reduction gear.

In JP-A-2001-270450, since the number of threads of the worm is set tothree, the number of contact teeth is increased and a contact pressureis reduced and thus the abrasion durability can be improved. Also, atooth bearing state of the three-thread worm is set forth in FIG. 7 ofJP-A-2001-270450, and a tooth bearing state (contact surfaces betweenthe worm and the wheel) of the two-thread worm is set forth in FIGS. 8,9 of JP-A-2001-270450. Also, as shown in FIGS. 45A, 45B (or FIGS. 46A,46B) of this application, JP-A-2001-270450 discloses respectivesituations that each contact surface extends in the tooth tracedirection and has a slight width in the tooth depth direction, and thecontact surface makes contact with the addendum side of the wheel in theinitial stage of the engagement and it makes contact with the dedendumside in the final stage of the engagement. In other words, theengagement makes progress while a line of contact in the tooth tracedirection is shifting sequentially from the addendum side to thededendum side.

In JP-A-2002-173041, since the wheel shape that can lengthen a line ofcontact between the cylindrical worm and the wheel tooth surface isemployed, the contact pressure is lowered and thus the abrasiondurability can be improved. Also, in JP-A-2002-173041, an improvement ismade to increase a tooth bearing area. Since the crowing is provided onboth tooth surfaces of the worm and the worm wheel in the tooth tracedirection, it can be prevented that the contact length in the toothtrace direction is shortened. That is, in JP-A-2002-173041, the contactportion that is extended in the tooth trace direction can be obtained incontrast to JP-A-2001-270450.

Also, in both JP-A-2001-270450 and JP-A-2002-173041, since a facepressure of the wheel gear made of the resin is lowered by widening thecontact area, and thus the abrasion durability can be improved.

In contrast, in JP-A-9-132154 that discloses the hourglass worm, theworm is formed as the hourglass worm that has a worm profile to trace anouter peripheral shape of the wheel. Thus, similarly the number ofworking teeth can be increased.

In the case of the hourglass worm the development of which is proceedingnowadays, a distance between the rotating shaft of the wheel and therotating shaft of the worm, both shafts being neither parallel norintersecting, is increased from the shortest distance, which is given asa length of the common perpendicular line to both rotation shafts(center-to-center distance) in accordance with a rotation phase of thewheel.

Then, an increment a of a pitch circle radius of the worm is given byσ=R−√{square root over (R ² −X ² )}  [Formula 1]where R is a pitch circle radius of the wheel, and

-   -   X is a distance from a common perpendicular line of the worm.

Therefore, the pitch circle diameter of the hourglass worm issymmetrically increased continuously as a position X goes away from theposition (X=0), on which the common perpendicular line is located and atwhich the minimum diameter is given, in the axial direction of the worm.

Meanwhile, as shown in FIG. 47, in the cylindrical worm, the cylindricalworm b is supported rotatably by the gear housing a. In this case, eventhough the cylindrical worm b is displaced with respect to the gearhousing a in the axial direction, such positional displacement has noinfluence on the engagement between the wheel c and the cylindrical wormb at all, for the pitch circle of the cylindrical worm b has a constantvalue at any position of the cylindrical worm b in the axial direction,as shown in FIG. 48B in an enlarged fashion.

Here, an enveloping surface obtained by connecting the pitch circlealong the axial direction gives a circular cylinder. The illustrationshows a sectional shape of the circular cylinder. An intersection pointbetween the cylindrical surface and the pitch circle of the wheel c isnot varied even when the cylindrical surface is shifted in the axialdirection. In the case of the cylindrical worm b, first the wheel c isincorporated into the gear housing a, and then the cylindrical worm bcan be fitted to the wheel c by screwing this worm b into the wheel cfrom the motor fitting hole g side while rotating it.

However, in the case of the hourglass worm, the position of the minimumpitch circle of the hourglass worm must be aligned very precisely withpositions of the common perpendicular line to the rotating shaft of thewheel and the rotating shaft of the worm in the gear housing. When thehourglass worm is displaced to the (−) side with respect to the wheel,both pitch circles go away from each other on one end side of thehourglass worm but both pitch circles intersect with each other on theother end side. Consequently, the backlash becomes large on one end sidebut the backlash becomes small on the other end side. In the case wherea change of the backlash due to the displacement is large, the smoothrotating transmission cannot be executed owing to the interference oftooth surfaces. Also, there exists such a problem that, when thebacklash becomes large, the touching sound between mutual tooth surfacesbecomes loud. As a result, the position of the hourglass worm in theaxial direction must be adjusted precisely.

Also, as shown in FIG. 49A, in the cylindrical worm, a bearing h thatbears rotatably the axial end side of the cylindrical worm b is fittedinto the gear housing a, and then the wheel c is fitted into the gearhousing a. Then, as shown in FIGS. 49B, 49C, the cylindrical worm b isfitted into the bearing h on the axial end side by screwing the worm binto the wheel from the motor fitting hole g side while rotating it, andthen a bearing f can be fitted to the motor fitting hole g side. As aresult, the assembling operation of the cylindrical worm is extremelyeasy.

However, in the case of the hourglass worm, the same fitting way as thecylindrical worm cannot be applied because of the interference betweenthe worm and the wheel. Therefore, the hourglass worm must be assembledtemporarily while avoiding the interference with the wheel, then thebearings that bear both ends of the hourglass worm are fitted from bothends, and then each position of the end surface of the bearing must beadjusted by a shim, or the like to correct the misalignment. As aresult, the assembling operation of the hourglass worm is troublesome.

Also, in the case where first the hourglass worm is fitted into thehousing and then the wheel is fitted thereto, the wheel profile must beformed into the shape (e.g., like the helical gear) that does notinterfere with the hourglass worm in the axial direction of the wheel.Therefore, the number of working teeth is increased during theengagement between the helical gear wheel and the hourglass worm,nevertheless respective tooth surfaces can mesh merely via a pointcontact and thus a contact pressure is increased. As a result, such aproblem exists that the abrasion durability cannot be improved asexpected.

Also, in the cylindrical worm, the pitch circle diameter can be measuredsimply by the three probe method after the machining process iscompleted.

However, in the hourglass worm, it is impossible to measure the pitchcircle by the conventional three probe method because the pitch circlediameter is changed continuously. Thus, it is difficult to detectprecisely the position of the minimum diameter of the pitch circle inthe axial direction, and then such position is decided depending on apositional precision deduced from the working reference applied inmachining the worm.

As described above, in order to correct the positional displacement(misalignment) caused due to the processing error of the hourglass wormand the housing, the troublesome operation to adjust precisely itsposition in the axial direction is required of the hourglass worm.

By the way, the conventional EPS worm reduction gear employs theinvolute tooth profile. When the engagement between the worm having theinvolute tooth profile and the worm wheel is viewed from the centerplane of the worm (the plane that is perpendicular to the wheel shaft tocontain the worm shaft), such engagement is equivalent to the engagementbetween the rack and the pinion (wheel), which appears on the sectionalshape of the worm shaft. Normals of both tooth profiles are common at apoint of contact of both tooth surfaces of the rack and the pinion (theworm and the wheel), and the normal is tangent to both base circlesowing to the definition of the involute. That is, like the case of thegear system with parallel axes, the engagement starts from a point atwhich the common tangent of both base circles and the tooth surfacesintersect with each other, and then moves from the addendum side to thededendum side.

The worm reduction gear is different from the gear system with parallelaxes resides in that, since the rack teeth proceed by the rotation ofthe worm, the engagement between the worm and the wheel is performed viathe sliding contact on the front surface of the worm.

The sliding contact of the worm and a line of contact in the prior artare directed in the almost same direction. Therefore, the lubricant isexhausted more easily out of the contact range by the rotation of theworm as a line of contact is extended longer in the tooth tracedirection.

In contrast, since the worm reduction gear utilizes the slidingtransmission, it is a common sense that normally such worm reductiongear is lubricated with oil and thus the lubricant is supplied all thetime. However, in the electric power steering, the grease is used as thelubricant for the reasons to improve the handling performance, preventthe contamination due to the oil leakage, prevent the deterioration ofthe steering feeling due to an increase in the sliding resistance of thesealing member (seal), and so on.

Therefore, according to the conventional approaches set forth inJP-A-2001-270450 and JP-A-2002-173041 to reduce the contact pressure,the desired effect can be achieved in a short term, but the lubricant iscarried out of the engaging range when the electric power steering isused over a long term. As a consequence, there existed such a problemthat the abrasion is caused abruptly owing to the lubrication failure.

DISCLOSURE OF THE INVENTION

The present invention has been made in view of the above circumstances,and it is a first object of the present invention to provide an electricpower steering capable of improving a contact ratio by using anhourglass worm to attain a higher output, and also performing acorrection of a misalignment easily by simplifying remarkably a fittingoperation of the hourglass worm.

Also, it is a second object of the present invention to provide anelectric power steering capable of improving a lubricating performanceby using a tooth profile with a special shape to improve considerablythe abrasion durability.

In addition, it is a third object of the present invention to provide anelectric power steering capable of improving the contact ratio by usingthe hourglass worm to attain the higher output, and also attaining thecorrection of the misalignment easily by simplifying remarkably apositioning operation of the hourglass worm.

In order to achieve the above first object, in an electric powersteering according to the present invention for generating an assiststeering torque from an electric motor in response to a steering torqueapplied to a steering wheel, and then transmitting the assist steeringtorque to an output shaft of a steering equipment after a speedreduction using a worm gear system, the worm gear system causes ahourglass worm driven by the electric motor to mesh with a worm wheelprovided to the output shaft, and at least one of bearings that bearrotatably the hourglass worm is composed of a tapered roller bearing, anangular contact bearing, or a magneto ball bearing, from which an outerring can be separated.

Also, in an electric power steering according to the present inventionfor generating an assist steering torque from an electric motor inresponse to a steering torque applied to a steering wheel, and thentransmitting the assist steering torque to an output shaft of a steeringequipment after a speed reduction using a worm gear system, the wormgear system causes a hourglass worm driven by the electric motor to meshwith a worm wheel provided to the output shaft, a bearing holder that isput onto an outer ring and has a taper surface on an outer peripheralsurface is provided to at least one of bearings that bear rotatably thehourglass worm, and a taper hole with which the taper surface of thebearing holder is engaged is formed in a gear housing.

Also, in an electric power steering according to the present inventionfor generating an assist steering torque from an electric motor inresponse to a steering torque applied to a steering wheel, and thentransmitting the assist steering torque to an output shaft of a steeringequipment after a speed reduction using a worm gear system, the wormgear system causes a hourglass worm driven by the electric motor to meshwith a worm wheel provided to the output shaft, a bearing holder that isfitted into an inner ring and has a taper surface on an inner peripheralsurface is provided to at least one of bearings that bear rotatably thehourglass worm, and a taper surface that is engaged with the tapersurface of the bearing holder is formed on a shaft end portion of thehourglass worm.

Also, in an electric power steering according to the present inventionfor generating an assist steering torque from an electric motor inresponse to a steering torque applied to a steering wheel, and thentransmitting the assist steering torque to an output shaft of a steeringequipment after a speed reduction using a worm gear system, the wormgear system causes a hourglass worm driven by the electric motor to meshwith a worm wheel provided to the output shaft, an inner peripheralsurface of an inner ring of at least one of bearings that bear rotatablythe hourglass worm is formed as a taper surface, and a taper surfacethat is engaged with the taper surface of the inner ring is formed onthe hourglass worm.

Also, in an electric power steering according to the present inventionfor generating an assist steering torque from an electric motor inresponse to a steering torque applied to a steering wheel, and thentransmitting the assist steering torque to an output shaft of a steeringequipment after a speed reduction using a worm gear system, the wormgear system causes a hourglass worm driven by the electric motor to meshwith a worm wheel provided to the output shaft, and at least one ofbearings that bear rotatably the worm wheel is composed of a taperedroller bearing, an angular contact bearing, or a magneto ball bearing,from which an outer ring can be separated.

Also, in an electric power steering according to the present inventionfor generating an assist steering torque from an electric motor inresponse to a steering torque applied to a steering wheel, and thentransmitting the assist steering torque to an output shaft of a steeringequipment after a speed reduction using a worm gear system, the wormgear system causes a hourglass worm driven by the electric motor to meshwith a worm wheel provided to the output shaft, a bearing holder that isput onto an outer ring and has a taper surface on an outer peripheralsurface is provided to at least one of bearings that bear rotatably theworm wheel, and a taper hole with which the taper surface of the bearingholder is engaged is formed in a gear housing.

Also, in an electric power steering according to the present inventionfor generating an assist steering torque from an electric motor inresponse to a steering torque applied to a steering wheel, and thentransmitting the assist steering torque to an output shaft of a steeringequipment after a speed reduction using a worm gear system, the wormgear system causes a hourglass worm driven by the electric motor to meshwith a worm wheel provided to the output shaft, and at least one ofbearings that bear rotatably the hourglass worm is provided such that aposition can be changed with respect to a gear housing in acenter-to-center direction.

Also, in order to achieve the above second object, in an electric powersteering according to the present invention for generating an assiststeering torque from an electric motor in response to a steering torqueapplied to a steering wheel, and then transmitting the assist steeringtorque to an output shaft of a steering equipment after a speedreduction using a worm gear system, the worm gear system causes ahourglass worm driven by the electric motor to mesh with a worm wheelprovided to the output shaft, and a tooth profile of the worm wheel anda tooth profile of the worm are shaped into a special tooth profile inwhich a first line of contact and a second line of contact, whichintersect with a sliding direction of the worm and intersect with eachother, and a tooth surface of an intermediate gear is formed as aconical surface.

In this case, preferably at least a dedendum shape of the worm should beformed as a hourglass shape.

Also, preferably a consistency of a grease should be set to 385 or less.

Also, preferably a width of the worm wheel should be formed wider than aminimum dedendum circle diameter of the hourglass worm.

Also, preferably a clearance on both end sides should be set larger thana clearance in a center portion of the worm wheel in a tooth tracedirection.

Also, preferably the electric motor consists of a brushless motor.

Also, in order to achieve the above third object, in an electric powersteering according to the present invention for generating an assiststeering torque from an electric motor in response to a steering torqueapplied to a steering wheel, and then transmitting the assist steeringtorque to an output shaft of a steering equipment after a speedreduction using a worm gear system, the worm gear system causes ahourglass worm driven by the electric motor to mesh with a worm wheelprovided to the output shaft.

Also, preferably a backlash on both end portions of the hourglass wormshould be set larger than a backlash in an engagement center portion ofthe hourglass worm.

Also, preferably a number of working teeth between the hourglass wormand the worm wheel should be increased in response to a transmissiontorque.

Also, preferably at least one of working teeth of the hourglass worm andthe worm wheel can be deformed elastically.

Also, preferably at least tooth portion of the worm wheel should beformed of resin material.

Also, preferably a number of threads of the hourglass worm should be setto 2 threads or more.

Also, preferably a tooth thickness adjusting process of reducing eachtooth thickness should be applied to the hourglass worm.

Also, preferably the tooth thickness adjusting process of the tooththickness adjusting process shapes a tooth profile such that a tooththickness is thinned toward both end portions from a center portion ofthe worm in an axial direction.

Also, preferably the tooth thickness adjusting process of the tooththickness adjusting process shapes the tooth profile such that theprocess is not applied to a predetermined interval of the center portionof the worm in the axial direction and the process is applied toremaining intervals either to reduce the tooth thickness toward both endportions or to form a constant tooth thickness that is thinner than thetooth thickness in an interval in which the process is not applied.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal sectional view of an electric power steeringaccording to an illustrative example of the present invention.

FIG. 2 is a longitudinal sectional view of an electric power steeringaccording to a first embodiment of the present invention.

FIGS. 3A to 3D are schematic views showing a fitting step of theelectric power steering according to the first embodiment respectively.

FIG. 4 is a longitudinal sectional view of an electric power steeringaccording to a second embodiment of the present invention.

FIG. 5 is a longitudinal sectional view of an electric power steeringaccording to a third embodiment of the present invention.

FIG. 6 is a longitudinal sectional view of an electric power steeringaccording to a fourth embodiment of the present invention.

FIG. 7 is a longitudinal sectional view of an electric power steeringaccording to a fifth embodiment of the present invention.

FIG. 8 is a longitudinal sectional view of an electric power steeringaccording to a sixth embodiment of the present invention.

FIG. 9 is a longitudinal sectional view of an electric power steeringaccording to a seventh embodiment of the present invention.

FIG. 10 is a longitudinal sectional view of an electric power steeringaccording to an eighth embodiment of the present invention.

FIG. 11A is a longitudinal sectional view of a column assist electricpower steering according to a ninth embodiment of the present invention,and FIG. 11B is a sectional view showing a pertinent portion of a wormgear system of the electric power steering.

FIG. 12A is a longitudinal sectional view of a column assist electricpower steering according to a tenth embodiment of the present invention,and FIG. 12B is a sectional view showing a pertinent portion of a wormgear system of the electric power steering.

FIG. 13 is a longitudinal sectional view of an electric power steeringaccording to an eleventh embodiment of the present invention.

FIG. 14 is a longitudinal sectional view of an electric power steeringaccording to a twelfth embodiment of the present invention.

FIG. 15 is a longitudinal sectional view of an electric power steeringaccording to a thirteenth embodiment of the present invention.

FIGS. 16A to 16C are schematic views showing a fitting step of theelectric power steering according to the thirteenth embodimentrespectively.

FIG. 17 is a longitudinal sectional view of an electric power steeringaccording to a fourteenth embodiment of the present invention.

FIG. 18 is a state diagram of a line of contact in the electric powersteering according to the fourteenth embodiment of the presentinvention.

FIG. 19 is a view showing relationships among a clearance and a wormroot diameter and a wheel tooth thickness in the electric power steeringaccording to the fourteenth embodiment of the present invention.

FIG. 20 is a longitudinal sectional view of an electric power steeringaccording to a fifteenth embodiment of the present invention.

FIG. 21A is a longitudinal sectional view of a column assist electricpower steering according to the present invention, and FIG. 21B is asectional view showing a pertinent portion of a worm gear system of theelectric power steering.

FIG. 22A is a front view containing a partially sectioned cross sectionof a pinion assist electric power steering according to the presentinvention, and FIG. 22B is a sectional view showing a pertinent portionof the electric power steering.

FIG. 23 is a longitudinal sectional view of an electric power steeringaccording to a sixteenth embodiment of the present invention.

FIG. 24A is a longitudinal sectional view of the electric power steeringshown in FIG. 23, and FIG. 24B is a schematic view showing arelationship between a pitch circle of an hourglass worm and a pitchcircle of a wheel.

FIG. 25A is a longitudinal sectional view of the electric power steeringshown in FIG. 23, FIG. 25B is a schematic view showing a relationshipbetween a pitch circle envelope of the hourglass worm and the pitchcircle of the wheel, and FIG. 25C is a schematic view showing an extentof a backlash.

FIG. 26A is a longitudinal sectional view showing an hourglass wormreduction gear in an electric power steering according to a seventeenthembodiment of the present invention, and FIG. 26B is an enlarged view ofan engaging portion.

FIG. 27 is an enlarged view showing an hourglass worm in FIG. 26A.

FIG. 28A is a longitudinal sectional view showing a reduction gear thatcontains an assembling error of the hourglass worm, to which a tooththickness adjusting process is applied, in the axial direction (+direction), and FIG. 28B is an enlarged view of an engaging portion.

FIG. 29A is a longitudinal sectional view showing a reduction gear thatcontains the assembling error of the hourglass worm, to which the tooththickness adjusting process is applied, in the axial direction (−direction), and FIG. 29B is an enlarged view of an engaging portion.

FIG. 30A is a longitudinal sectional view showing an hourglass wormreduction gear in an electric power steering according to an eighteenthembodiment of the present invention, and FIG. 30B is an enlarged view ofan engaging portion.

FIG. 31 is an enlarged view showing an hourglass worm in FIG. 30A.

FIG. 32A is a graph showing a relationship between a tooth thickness ofa worm and an angle from a center of the worm wheel, and FIG. 32B is aview explaining the graph in FIG. 32A.

FIG. 33A is a longitudinal sectional view showing a reduction gear thatcontains an assembling error of the hourglass worm, to which the tooththickness adjusting process is applied, in the axial direction (+direction), and FIG. 33B is an enlarged view of an engaging portion.

FIG. 34A is a longitudinal sectional view showing a reduction gear thatcontains the assembling error of the hourglass worm, to which the tooththickness adjusting process is applied, in the axial direction (−direction), and FIG. 34B is an enlarged view of an engaging portion.

FIG. 35 is an explanatory view showing an engagement between thehourglass worm, to which the tooth thickness adjusting process isapplied, and the worm wheel in the small torque transmission.

FIG. 36 is an explanatory view showing the engagement between thehourglass worm, to which the tooth thickness adjusting process isapplied, and the worm wheel in the large torque transmission.

FIG. 37A is a longitudinal sectional view showing an hourglass wormreduction gear in an electric power steering according to a nineteenthembodiment of the present invention, and FIG. 37B is an enlarged view ofan engaging portion.

FIG. 38 is an enlarged view showing an hourglass worm in FIG. 37A.

FIG. 39A is a graph showing a relationship between a tooth thickness ofa worm and an angle wheel from a center of the worm, and FIG. 39B is aview explaining the graph in FIG. 39A.

FIG. 40A is a longitudinal sectional view showing a reduction gear thatcontains an assembling error of the hourglass worm, to which the tooththickness adjusting process is applied, in the axial direction (+direction), and FIG. 40B is an enlarged view of an engaging portion.

FIG. 41A is a longitudinal sectional view showing a reduction gear thatcontains the assembling error of the hourglass worm, to which the tooththickness adjusting process is applied, in the axial direction (−direction), and FIG. 34B is an enlarged view of an engaging portion.

FIG. 42A is a longitudinal sectional view showing a hourglass wormreduction gear, to which the tooth thickness adjusting process is notapplied, in an electric power steering, and FIG. 42B is an enlarged viewof an engaging portion.

FIG. 43A is a longitudinal sectional view showing a reduction gear thatcontains an assembling error of the hourglass worm, to which the tooththickness adjusting process is not applied, in the axial direction (+direction), and FIG. 43B is an enlarged view of an engaging portion.

FIG. 44A is a longitudinal sectional view showing a reduction gear thatcontains the assembling error of the hourglass worm, to which the tooththickness adjusting process is not applied, in the axial direction (−direction), and FIG. 44B is an enlarged view of an engaging portion.

FIGS. 45A and 45B are state diagrams showing a line of contact in anelectric power steering shown in FIG. 45C in the prior art respectively,and FIG. 45C is a longitudinal sectional view of the electric powersteering in the prior art.

FIGS. 46A and 46B are state diagrams showing a line of contact in theelectric power steering shown in FIG. 45C in the prior art respectively.

FIG. 47 is a longitudinal sectional view of the electric power steeringin the prior art.

FIG. 48A is a longitudinal sectional view of the electric power steeringshown in FIG. 47, and FIG. 48B is a schematic view showing arelationship between a pitch circle of a cylindrical worm and a pitchcircle of a wheel.

FIGS. 49A to 49C are schematic views showing a fitting step of theelectric power steering shown in FIG. 46 respectively.

In Figures, a reference numeral 1 denotes a gear housing, 2 hourglassworm, 3 worm wheel, 4 electric motor, 5 output shaft, 5 a torsion bar, 6bearing, 7 bearing (tapered roller bearing, or the like), 7 a innerring, 7 b rolling element, 7 c outer ring, 7 d taper surface, 8 snapring, S shim, 10 motor fitting hole, 11 bearing holder, 11 a tapersurface, 12 taper hole, 13 bearing holder, 13 a taper surface, 14 tapersurface, 15 preload-adjusting threaded member, 16 fixing nut, 17 preloadadjusting plate, 18 bolt, 19 nut, 21 bearing, 22 bearing (tapered rollerbearing, or the like), 22 a inner ring, 22 b rolling element, 22 c outerring, 23 bearing holder, 23 a taper surface, 24 taper hole, 31longitudinal-movement adjusting threaded member, 32 nut, 33 fitting holeon the shaft end side, 41 center-to-center distance adjusting member, 42elastic body such as spring, rubber, resin, and the like, 43 screwmember, 44 O ring, 201 gear housing, 202 hourglass worm, 203 worm wheel,204 electric motor, 205 output shaft, 205 a torsion bar, 206 bearing,207 bearing (tapered roller bearing, or the like), 208 snap ring, 20Sshim, 209 cover, 210 motor fitting hole, 220 worm gear housing, 501 gearhousing, 502 hourglass worm, 503 worm wheel, 504 electric motor, 505output shaft, 505 a torsion bar, 506 bearing, 507 bearing, 508 snapring, 509 cover, 510 motor fitting hole.

BEST MODE FOR CARRYING OUT THE INVENTION

Electric power steerings according to embodiments of the presentinvention will be explained with reference to the drawings hereinafter.

Illustrative Example

FIG. 1 is a longitudinal sectional view of an electric power steeringaccording to an illustrative example of the present invention.

In this illustrative example, an hourglass worm 2 and a worm wheel 3that is meshed with the hourglass worm 2 are installed into a gearhousing 1 of the worm gear system, and an electric motor 4 for drivingthe hourglass worm 2 is mounted on the side of the gear housing 1. Theworm wheel 3 is fitted onto an output shaft 5 (e.g., a pinion shaft or acolumn shaft) of the steering equipment. Accordingly, an assist steeringtorque is generated from the electric motor 4 in response to thesteering torque applied to a steering wheel (not shown), and thentransmitted to the output shaft 5 of the steering equipment via areduction gear that consists of the hourglass worm 2 and the wheel 3.Here, a reference numeral 5 a denotes a torsion bar.

In this illustrative example, in the case of the hourglass worm 2, thesame fitting way as the cylindrical worm cannot be applied because thepitch circles interfere with each other. For this reason, bearings 6, 7are fitted from both end sides in the situation that the hourglass worm2 is meshed with the wheel 3. More particularly, the bearings 6, 7 forbearing rotatably both end portions of the hourglass worm 2 are fittedadjustably by a shim S and a cover 9 respectively. Thus, themisalignment can be corrected by adjusting end face positions of thebearings 6, 7 by an end face position of the shim S or the cover 9, etc.

However, in the present illustrative example, it is very troublesome tocorrect the misalignment of the bearings 6, 7 from both ends of thehourglass worm 2 because there are many adjusting margins and adjustedportions, and thus the fitting operation becomes difficult.

Here, a reference numeral 8 denotes a snap ring. This is common in allembodiments described in the following.

First Embodiment

FIG. 2 is a longitudinal sectional view of an electric power steeringaccording to a first embodiment of the present invention. FIGS. 3A to 3Dare schematic views showing a fitting step of the electric powersteering according to the first embodiment respectively.

In the first embodiment, the hourglass worm 2 and the worm wheel 3 thatis engaged with the hourglass worm 2 are installed into the gear housing1 of the worm gear system, and the electric motor 4 for driving thehourglass worm 2 is mounted on the side of the gear housing 1. The wormwheel 3 is fitted onto the output shaft 5 (e.g., the pinion shaft or thecolumn shaft) of the steering equipment. Accordingly, the assiststeering torque is generated from the electric motor 4 in response tothe steering torque applied to the steering wheel (not shown), and thentransmitted the output shaft 5 of the steering equipment via thereduction gear that consists of the hourglass worm 2 and the wheel 3.Here, the reference numeral 5 a denotes a torsion bar.

The bearing 6 for bearing the motor-side end portion of the hourglassworm 2 is composed of the ball bearing, and is fitted such that itsposition can be adjusted by using the shim S. In contrast, the bearingfor bearing the shaft end portion of the hourglass worm 2 is composed ofthe tapered roller bearing 7 from which an outer ring 7 c can beseparated and to which both a radial force and a thrust force can beapplied.

In the fitting operation, as shown in FIG. 3A, an inner ring 7 a androlling elements 7 b are fitted to the hourglass worm 2, while the outerring 7 c is fitted in advance to the gear housing 1.

Then, as shown in FIGS. 3B to 3D, the hourglass worm is moved obliquelyto the rotating shaft line of the hourglass worm 2 in the gear housing 1along the raceway of the outer ring 7 c. Thus, the tapered rollerbearing 7 is assembled in the gear housing 1.

In this manner, the fitting portion of the tapered roller bearing 7 onthe shaft end side is manufactured as a reference portion to eliminatethe positional adjustment, and the preload is applied by the bearing 6on the motor fitting hole 10 side. This preload adjustment is executedby using the shim S.

In other words, the fitting of the hourglass worm 2 into the bearing 7(tapered roller bearing) on the shaft end side is performed in theoblique direction to the rotating shaft line of the hourglass worm 2.Therefore, the hourglass worm 2 can be fitted from the motor fittinghole 10 side, and also the misalignment can be corrected from one side.

With the above, the contact ratio can be improved by using the hourglassworm 2 to achieve the higher output. Also, the misalignment can becorrected easily by making the fitting operation of the hourglass worm 2remarkably easy.

Second Embodiment

FIG. 4 is a longitudinal sectional view of an electric power steeringaccording to a second embodiment of the present invention.

In the second embodiment, the bearing 6 for bearing the motor-side endportion of the hourglass worm 2 is composed of the ball bearing, and isfitted such that its position can be adjusted by using the shim S. Incontrast, the bearing for bearing the shaft end portion of the hourglassworm 2 is composed of the angular contact bearing 7 from which the outerring 7 c can be separated and which can support both the radial forceand the thrust force.

In the fitting operation, the inner ring 7 a and the rolling elements 7b are fitted to the hourglass worm 2, while the outer ring 7 c is fittedbeforehand to the gear housing 1. Then, the hourglass worm 2 is movedobliquely to the rotating shaft line of the hourglass worm 2 in the gearhousing 1 along the raceway of the outer ring 7 c. Thus, the angularcontact bearing 7 is assembled in the gear housing 1.

In this manner, the fitting portion of the angular contact bearing 7 onthe shaft end side is manufactured as a reference portion to eliminatethe positional adjustment, and the preload is applied by the bearing 6on the motor fitting hole 10 side. This preload adjustment is carriedout by using the shim S. In other words, the fitting of the hourglassworm 2 into the bearing 7 (angular contact bearing) on the shaft endside is applied in the oblique direction to the rotating shaft line ofthe hourglass worm 2. Therefore, the hourglass worm 2 can be fitted fromthe motor fitting hole 10 side, and also the misalignment can becorrected from one side. With the above, the contact ratio can beimproved by using the hourglass worm 2 to achieve the higher output.Also, the misalignment can be corrected easily by facilitatingremarkably the fitting operation of the hourglass worm 2.

Third Embodiment

FIG. 5 is a longitudinal sectional view of an electric power steeringaccording to a third embodiment of the present invention.

In the third embodiment, the bearing provided on the shaft end side ofthe hourglass worm 2 is composed of the deep groove ball bearing 7, anda cylindrical bearing holder 11 having a taper surface 11 a on its outerperipheral surface is put on the outer ring 7 c of the deep groove ballbearing 7.

A taper hole 12 with which the taper surface 11 a of the bearing holder11 is engaged is formed in the end portion of the gear housing 1.

Therefore, in the fitting operation of the hourglass worm 2, the bearingholder 11 is inserted while sliding the taper surface 11 a of thebearing holder 11 along the taper hole 12 of the gear housing 1. Inother words, the fitting of the hourglass worm 2 into the bearing 7(deep groove ball bearing) on the shaft end side is applied in theoblique direction to the rotating shaft line of the hourglass worm 2.Therefore, the hourglass worm 2 can be fitted from the motor fittinghole 10 side, and also the misalignment can be corrected from one side.With the above, not only the contact ratio can be improved by using thehourglass worm 2 to achieve the higher output, but also the misalignmentcan be corrected readily by simplifying remarkably the fitting operationof the hourglass worm 2.

Fourth Embodiment

FIG. 6 is a longitudinal sectional view of an electric power steeringaccording to a fourth embodiment of the present invention.

In the fourth embodiment, the bearing provided on the shaft end side ofthe hourglass worm 2 is composed of the deep groove ball bearing 7, andthe cylindrical bearing holder 11 is put on the outer ring 7 c of thedeep groove ball bearing 7. In this case, the taper surface 11 a of thebearing holder 11 is protruded from a substantially central portion ofthe bearing holder 11 in the axial direction.

The taper hole 12 with which the taper surface 11 a of the bearingholder 11 is engaged is formed in the end portion of the gear housing 1.

Therefore, in the fitting operation of the hourglass worm 2, the bearingholder 11 is inserted while sliding the taper surface 11 a of thebearing holder 11 along the taper hole 12 of the gear housing 1. Inother words, the fitting of the hourglass worm 2 into the bearing 7(deep groove ball bearing) on the shaft end side is applied in theoblique direction to the rotating shaft line of the hourglass worm 2.Therefore, the hourglass worm 2 can be fitted from the motor fittinghole 10 side, and also the misalignment can be corrected from one side.With the above, the contact ratio can be improved by using the hourglassworm 2 to achieve the higher output. Also, the misalignment can becorrected easily by making the fitting operation of the hourglass worm 2considerably easy.

Fifth Embodiment

FIG. 7 is a longitudinal sectional view of an electric power steeringaccording to a fifth embodiment of the present invention.

In the fifth embodiment, the bearing provided on the shaft end side ofthe hourglass worm 2 is composed of the deep groove ball bearing 7, anda cylindrical bearing holder 13 (bush) is fitted into the inner ring 7 aof the deep groove ball bearing 7. In this case, a taper surface 13 a isformed on an inner peripheral surface of the bearing holder 13 (bush).

A taper surface 14 that is engaged with the taper surface 13 a of thebearing holder 13 (bush) is formed on the shaft end portion of thehourglass worm 2.

Therefore, in the fitting operation of the hourglass worm 2, the deepgroove ball bearing 7 and the bearing holder 13 (bush) are installed inadvance into the gear housing 1, and then the hourglass worm 2 isinserted while sliding the taper surface 14 of the hourglass worm 2along the taper surface 13 a of the bearing holder 13 (bush).

In other words, the fitting of the hourglass worm 2 into the bearing 7(deep groove ball bearing) on the shaft end side is executed in theoblique direction to the rotating shaft line of the hourglass worm 2.Therefore, the hourglass worm 2 can be fitted from the motor fittinghole 10 side, and also the misalignment can be corrected from one side.With the above, the contact ratio can be improved by using the hourglassworm 2 to get the higher output. Also, the misalignment can be correctedeasily by facilitating considerably the fitting operation of thehourglass worm 2.

Sixth Embodiment

FIG. 8 is a longitudinal sectional view of an electric power steeringaccording to a sixth embodiment of the present invention.

In the sixth embodiment, the bearing provided on the shaft end side ofthe hourglass worm 2 is composed of the deep groove ball bearing 7, anda taper surface 7 d is formed on the inner ring 7 a of the deep grooveball bearing 7.

The taper surface 14 that is engaged with the taper surface 7 d of theinner ring 7 a of the deep groove ball bearing 7 is formed on the shaftend portion of the hourglass worm 2.

Therefore, in the fitting operation of the hourglass worm 2, the deepgroove ball bearing 7 is installed previously into the gear housing 1,and then the hourglass worm 2 is inserted while sliding the tapersurface 14 of the hourglass worm 2 along the taper surface 7 d of theinner ring 7 a of the deep groove ball bearing 7.

In other words, the fitting of the hourglass worm 2 into the bearing 7(deep groove ball bearing) on the shaft end side is applied in theoblique direction to the rotating shaft line of the hourglass worm 2.Therefore, the hourglass worm 2 can be fitted from the motor fittinghole 10 side, and also the misalignment can be corrected from one side.With the above, the contact ratio can be improved by using the hourglassworm 2 to achieve the higher output. Also, the misalignment can becorrected easily by making the fitting operation of the hourglass worm 2conspicuously easy.

Seventh Embodiment

FIG. 9 is a longitudinal sectional view of an electric power steeringaccording to a seventh embodiment of the present invention.

In the seventh embodiment, the bearing provided on the shaft end side ofthe hourglass worm 2 is composed of the deep groove ball bearing 7, andthe cylindrical bearing holder 13 (bush) is fitted into the inner ring 7a of the deep groove ball bearing 7. In this case, the taper surface 13a is formed on the inner peripheral surface of the bearing holder 13(bush).

The taper surface 14 that is engaged with the taper surface 13 a of thebearing holder 13 (bush) is formed on the shaft end portion of thehourglass worm 2.

Therefore, in the fitting operation of the hourglass worm 2, the deepgroove ball bearing 7 and the bearing holder 13 (bush) are installedpreviously into the gear housing 1, and then the hourglass worm 2 isinserted while sliding the taper surface 14 of the hourglass worm 2along the taper hole 13 a of the bearing holder 13 (bush).

In other words, the fitting of the hourglass worm 2 into the bearing 7(deep groove ball bearing) on the shaft end side is applied in theoblique direction to the rotating shaft line of the hourglass worm 2.Therefore, the hourglass worm 2 can be fitted from the motor fittinghole 10 side, and also the misalignment can be corrected from one side.

Further, in the seventh embodiment, a preload-adjusting threaded member15 is provided in the motor fitting hole 10 in such a manner that thethreaded member is screwed into the gear housing 1 to push the bearing6. A fixing nut 16 is screwed on the preload-adjusting threaded member15.

The preload applied to the bearing 6 on the motor side can be adjustedby this preload-adjusting threaded member 15.

With the above, not only the contact ratio can be improved by using thehourglass worm 2 to achieve the higher output, but also the misalignmentcan be corrected easily by simplifying considerably the fittingoperation of the hourglass worm 2.

Eighth Embodiment

FIG. 10 is a longitudinal sectional view of an electric power steeringaccording to an eighth embodiment of the present invention.

In the eighth embodiment, the bearing provided on the shaft end side ofthe hourglass worm 2 is composed of the deep groove ball bearing 7, andthe cylindrical bearing holder 13 (bush) is fitted into the inner ring 7a of the deep groove ball bearing 7. In this case, the taper surface 13a is formed on the inner peripheral surface of the bearing holder 13(bush).

The taper surface 14 that is engaged with the taper surface 13 a of thebearing holder 13 (bush) is formed on the shaft end portion of thehourglass worm 2.

Therefore, in the fitting operation of the hourglass worm 2, the deepgroove ball bearing 7 and the bearing holder 13 (bush) are installedpreviously into the gear housing 1, and then the hourglass worm 2 isinserted while sliding the taper surface 14 of the hourglass worm 2along the taper hole 13 a of the bearing holder 13 (bush).

In other words, the fitting of the hourglass worm 2 into the bearing 7(deep groove ball bearing) on the shaft end side is applied in theoblique direction to the rotating shaft line of the hourglass worm 2.Therefore, the hourglass worm 2 can be fitted from the motor fittinghole 10 side, and also the misalignment can be corrected from one side.

Further, in the eighth embodiment, a preload adjusting mechanism isprovided in the deep groove ball bearing 7 on the shat end side. Asshown in FIG. 10, for example, this preload adjusting mechanism consistsof a preload adjusting plate 17 for adjusting the preload of the deepgroove ball bearing 7, a bolt 18 for pushing the preload adjusting plate17, and a nut 19 screwed on the bolt 18.

The preload applied to the deep groove ball bearing 7 on the shaft endside can be adjusted by this preload adjusting mechanism.

With the above, the contact ratio can be improved by using the hourglassworm 2 to achieve the higher output. Also, the misalignment can becorrected easily by making the fitting operation of the hourglass worm 2considerably easy.

Ninth Embodiment

FIG. 1A is a longitudinal sectional view of a column assist electricpower steering according to a ninth embodiment of the present invention,and FIG. 11B is a sectional view showing a pertinent portion of a wormgear system of the electric power steering.

In the column assist electric power steering shown in FIG. 11A, a lowercolumn 102 is fitted into an upper column 101 of the steering column onthe front side of the vehicle, and an upper shaft 103 and a lower shaft104 (input shaft), both being spline-coupled, of the steering shaft aresupported rotatably in these columns 101, 102.

The output shaft 5 is coupled to the lower shaft 104 (input shaft) onthe front side of the vehicle. The steering gear (not shown) is coupledto the output shaft 5 on the front side of the vehicle via the universaljoint (not shown), or the like.

A base end of the torsion bar 5 a is press-fitted and fixed to the lowershaft 104 (input shaft) on the front side of the vehicle. The torsionbar 5 a is extended through a hollow inner side of the output shaft 5,and its top end is fixed to an end portion of the output shaft 5 by afixing pin 112.

A groove 113 for a torque sensor is formed in the output shaft 5 on therear side of the vehicle, and a sleeve 114 of the torque sensor isarranged on the outside of the groove 113 in the diameter direction. Aside end portion of this sleeve 114 on the rear side of the vehicle isfixed to the side end portion of the lower shaft 104 (input shaft) onthe front side of the vehicle by the caulking, or the like. A coil 115,a substrate, etc. are provided on the outside of the sleeve 114 in thediameter direction.

The worm wheel 3 that is meshed with the hourglass worm 2 as the drivingshaft of the electric motor 4 is fitted to the output shaft 5.

Therefore, a steering force generated when the driver turns a steeringwheel (not shown) can be transmitted to the turning wheel (not shown) ofthe vehicle via the input shaft 104, the torsion bar 5 a, the outputshaft 5, and the rack-and-pinion steering equipment. Also, a turningforce of the electric motor 4 is transmitted to the output shaft 5 viathe hourglass worm 2 and the worm wheel 3. Thus, the assist steeringtorque can be applied to the output shaft 5 by controlling appropriatelythe turning force and the turning direction of the electric motor 4.

In the ninth embodiment, one bearing 21 for bearing the output shaft 5(wheel 3) is composed of the ball bearing, and the other bearing 22 forbearing the output shaft 5 (wheel 3) is composed of the tapered rollerbearing 22 from which an outer ring 22 c can be separated and which cansupports both the radial force and the thrust force.

In the fitting operation, an inner ring 22 a and rolling elements 22 bare fitted onto the output shaft 5 (wheel 3), and the outer ring 22 c isfitted previously into the gear housing 1.

Then, the output shaft 5 (wheel 3) is moved obliquely to the rotatingshaft line of the output shaft 5 (wheel 3) in the gear housing 1 alongthe raceway of the outer ring 22 c. Thus, the tapered roller bearing 22is assembled in the gear housing 1.

Here the angular contact bearing or the magneto ball bearing, from whichits outer ring can be separated, may be employed in place of the taperedroller bearing 22.

Tenth Embodiment

FIG. 12A is a longitudinal sectional view of a column assist electricpower steering according to a tenth embodiment of the present invention,and FIG. 12B is a sectional view showing a pertinent portion of a wormgear system of the electric power steering.

In the tenth embodiment, the other bearing 22 for bearing the outputshaft 5 (wheel 3) is composed of the deep groove ball bearing 22, and acylindrical bearing holder 23 having a taper surface 23 a on its outerperipheral surface is put on the outer ring 22 c of the deep groove ballbearing 22.

A taper hole 24 with which the taper surface 23 a of the bearing holder23 is engaged is formed in the gear housing 1.

Therefore, in the fitting operation of the output shaft 5 (wheel 3), theoutput shaft 5 (wheel 3) is inserted while sliding the taper surface 23a of the bearing holder 23 along the taper hole 24 in the gear housing1. In other words, the fitting of the output shaft 5 (wheel 3) into thebearing 22 (deep groove ball bearing) can be applied in the obliquedirection to the rotating shaft line of the output shaft 5 (wheel 3).

Eleventh Embodiment

FIG. 13 is a longitudinal sectional view of an electric power steeringaccording to an eleventh embodiment of the present invention.

In the conventional configuration in regard to JP-A-9-132154, since thewheel cannot help being fitted after the hourglass worm was assembled,the wheel must be shaped into the helical gear profile to eliminate theinterference caused in the fitting operation. Although the number ofworking teeth is increased by employing the hourglass worm, thehourglass worm comes into contact with the wheel via a point contact. Asa consequence, an effect of increasing a contact area could not besufficiently achieved.

In light of such situation, in the eleventh embodiment, a double-bearingstructure that can support the radial load and the bilateral thrust loadis employed as the bearing 6 of the hourglass worm 2 on the motor side,and also such structure can be adjusted to move back and forth in theaxial direction. More particularly, as shown in FIG. 13, alongitudinal-movement adjusting threaded member 31 is provided such thatthe member is screwed into the gear housing 1 to hold two bearings 6therein. A nut 32 is screwed on the hourglass worm 2 side.

In contrast, the bearing 7 on the shaft end side is composed of aone-end-closed needle roller bearing. The bearing can be fitted into afitting hole 33 provided in the end portion of the gear housing 1 fromthe outside of the gear housing 1 to seal the gear housing tightly.

With the above, the contact ratio can be improved by using the hourglassworm 2 to attain the higher output. Also, the misalignment can becorrected readily by simplifying considerably the fitting operation ofthe hourglass worm 2.

Twelfth Embodiment

FIG. 14 is a longitudinal sectional view of an electric power steeringaccording to a twelfth embodiment of the present invention.

In the twelfth embodiment, a four-point-contact ball bearing that doesnot need the preload is provided as the bearing 6 of the hourglass worm2 on the motor side. Thus, the positional adjustment can be eliminated.

In contrast, the bearing 7 on the shaft end side is composed of theone-end-closed needle roller bearing. The bearing can be fitted into thefitting hole 33 provided in the end portion of the gear housing 1 fromthe outside of the gear housing 1 to seal the gear housing tightly.

With the above, not only the contact ratio can be improved by using thehourglass worm 2 to attain the higher output, but also the misalignmentcan be corrected readily by making the fitting operation of thehourglass worm 2 remarkably easy.

Thirteenth Embodiment

FIG. 15 is a longitudinal sectional view of an electric power steeringaccording to a thirteenth embodiment of the present invention. FIGS. 16Ato 16C are schematic views showing a fitting step of the electric powersteering according to the thirteenth embodiment respectively.

The thirteenth embodiment is characterized in that the bearing 7 on theshaft end side is provided such that its position can be adjusted withrespect to the gear housing 1 in the center-to-center direction.

More particularly, the bearing 7 on the shaft end side is composed ofthe one-end-closed needle roller bearing. A center-to-center distanceadjusting member 41 is fitted on the one-end-closed needle rollerbearing 7. A screw member 43 is screwed into the gear housing 1 to pushthe center-to-center distance adjusting member 41 via an elastic body 42such as a spring, a rubber, a resin, or the like. Accordingly, theone-end-closed needle roller bearing 7 and the center-to-center distanceadjusting member 41 are energized elastically toward the wheel 3 side.

In the fitting operation, as shown in FIG. 16A, the one-end-closedneedle roller bearing 7 is fitted onto the hourglass worm 2, then thecenter-to-center distance adjusting member 41 is fitted on theone-end-closed needle roller bearing 7, and then the hourglass worm 2,and the like are inserted into the gear housing 1 in a state that thecenter-to-center distance is kept sufficiently large not to interferewith the wheel 3.

Then, as shown in FIG. 16B, the center-to-center distance adjustingmember 41 is pushed into the gear housing 1. Thus, the hourglass worm 2and the one-end-closed needle roller bearing 7 are moved toward thewheel 3 in their engaging position, and then fitted. At the same time,the bearing 6 on the motor side is also fitted. Finally, as shown inFIG. 16C, the screw member 43 is screwed.

In this manner, the one-end-closed needle roller bearing 7 on the shaftend side of the hourglass worm 2 is set such that the bearing can bemoved with respect to the gear housing 1 in the wheel 3 direction. Thus,the hourglass worm 2 can be brought close to the wheel 3 side after suchhourglass worm 2 is fitted into the one-end-closed needle roller bearing7. As a result, the bearing 7 on the shaft end side can be provided insuch a way that its position can be adjusted with respect to the gearhousing 1 in the center-to-center direction.

With the above, the contact ratio can be improved by using the hourglassworm 2 to achieve the higher output. Also, the misalignment can becorrected easily by simplifying considerably the fitting operation ofthe hourglass worm 2.

Here a buffering O ring 44 is provided between the center-to-centerdistance adjusting member 41 and the screw member 43.

In this case, the present invention is not limited to the aboveembodiments, and the present invention may be varied variously. Morespecifically, the magneto ball bearing may be employed instead of thetapered roller bearing or the angular contact bearing.

Fourteenth Embodiment

FIG. 17 is a longitudinal sectional view of an electric power steeringaccording to a fourteenth embodiment of the present invention.

FIG. 18 is a state diagram of a line of contact in the electric powersteering according to the fourteenth embodiment of the presentinvention.

FIG. 19 is a view showing relationships among a clearance and a wormroot diameter and a wheel tooth thickness in the electric power steeringaccording to the fourteenth embodiment of the present invention.

In the fourteenth embodiment, a hourglass worm 202 and a worm wheel 203that is meshed with this hourglass worm 202 are installed into a gearhousing 201 of the worm gear system, and an electric motor 204 fordriving the hourglass worm 202 is attached to the side portion of thegear housing 201. The worm wheel 203 is fitted onto an output shaft 205(e.g., a pinion shaft or a column shaft) of the steering equipment.Therefore, the assist steering torque is generated from the electricmotor 204 in response to the steering torque applied to the steeringwheel (not shown), and then is transmitted to the output shaft 205 ofthe steering equipment via the reduction gear that consists of thehourglass worm 202 and a worm wheel 203. In this case, a referencesymbol 205 a denotes a torsion bar.

Also, owing to the interference of the pitch circles, the hourglass worm202 cannot be fitted in the same way as the cylindrical worm. Therefore,bearings 206, 207 are fitted from both end sides in the situation thatthe hourglass worm 202 is meshed with the worm wheel 203. In otherwords, the bearings 206, 207 that bear rotatably both end portions ofthe hourglass worm 202 are fitted such that their positions can beadjusted by using a shim 20S (motor fitting hole 210 side) and a cover209 (shaft end side) respectively. The misalignment can be corrected byadjusting the end surface positions of the bearings 206, 207 by virtueof the end surface positions of the shim 20S and the cover 209, or thelike. Here a reference symbol 208 denotes a snap ring. This is common inall embodiments described in the following.

In the fourteenth embodiment, as shown in FIG. 18, the tooth profiles ofthe worm 202 and the wheel 203 are modified from the involute toothprofiles to special tooth profiles. According to the special toothprofiles, the tooth surface of the wheel 203 and the tooth surface ofthe worm 202 contact mutually at two locations of a first line ofcontact and a second line of contact, which intersect with each otherand intersect with the sliding direction of the worm 202, in the toothtrace direction of the wheel 203, and a tooth surface of an intermediategear is shaped into the conical surface.

As the worm reduction gear having this tooth profile, there are theproduct (trademark: HIDECON) of Sumitomo Heavy Industries, Ltd., theproduct (trademark: HICRA) of Shin-Ei Manufacturing Co., Ltd, and soforth. These products are used in general industries and heavyconstruction machinery applications, and the oil lubrication is applied.

In the engagement of these tooth profiles, the line of contact appearson both end sides in the tooth trace direction of the wheel 203 and onthe addendum side in the tooth depth direction at the start time ofengagement, while the line of contact moves to the center portion in thetooth trace direction of the wheel 203 and on the dedendum side in thetooth depth direction at the end time of engagement.

A point at which two lines of contact intersect with each other gives alimit normal point, and a line obtained by connecting these points givesa limit normal point curve.

The tooth profiles can mesh with each other such that the grease as thelubricant is gathered up toward the limit normal point curve near thecenter in the tooth trace direction according to two lines of contact.Therefore, the lubricant is not carried out of the wheel 203, and mostof the lubricant can be held within the face width of the wheel 203.

As a result, in the electric power steering to which the lubricant isnot supplied in the course of use, it is possible to prevent thedegradation of the durability due to the lubrication failure in along-term use.

In order to provide a line of contact directed in the tooth depthdirection that intersects with the sliding direction of the worm 202,the worm takes the hourglass profile since a pressure angle of the toothsurface of the worm 202 to the rotating shaft of the worm 202 is changedcontinuously according to the rotating position of the wheel 203.

Consequently, the number of simultaneously contacting/working teeth canbe increased, and an effect of reducing the face pressure can beachieved at the same time like the prior art, and also an oil filmnecessary for the lubrication can be thinned. As a result, such effectscan be further enhanced.

Also, if the grease having a poor fluidity and a consistency of 385 orless is employed, such effects can also be further enhanced.

In addition, the grease that is gathered up to the dedendum side in thecenter of the face width of the wheel 203 by an action of the line ofcontact is carried to both end sides of the wheel 203 by a relativesliding motion between the top of the worm 202 and the bottom of thewheel 203 generated by the rotation of the top of the worm 202. Then,the grease is returned to the addendum side by the rotation of the wheel203 to circulate.

However, as shown in FIG. 18, if a clearance between the top of the worm202 and the bottom of the wheel 203 is set constant, an amount of thegrease that is carried out of the tooth surface of the wheel 203 by therotational motion of the worm 202 is increased due to the viscosity ofthe grease.

Therefore, as shown in FIG. 19, if the clearances (δ1, δ2) are increasedtoward the end portion of the wheel 203, a moving power of the greasedue to the viscous resistance can be lessened toward both ends of thewheel 203. Therefore, an amount of the grease that is carried out of theface width of the wheel 203 can be reduced, and thus the grease can beretained more effectively within the face width of the wheel 203.

Also, in order to increase an amount of the grease that can be heldwithin the face width of the wheel 203 (to decrease an amount of thegrease that is carried out of the wheel 203), it is desired that theface width of the wheel 203 should be set larger than a minimum toothspace diameter of the worm 202.

Fifteenth Embodiment

FIG. 20 is a longitudinal sectional view of an electric power steeringaccording to a fifteenth embodiment of the present invention.

In contrast to the fourteenth embodiment, in the fifteenth embodiment,as shown in FIG. 20, the top side of a worm 220 is formed as acylindrical shape.

In the fourteenth embodiment, both ends of the hourglass worm 202 areincreased in diameter and the gear housing 201 is increased in size, andthus the fitting performance becomes worse. Also, the circulation of thegrease becomes difficult toward both ends of the tooth trace directionof the wheel 203 and on the top side.

However, in the fifteenth embodiment, as shown in FIG. 20, the top sideof the worm 220 is formed as the cylindrical shape. Therefore, theengagement on both ends of the wheel 203 and on the top side can belessened, and thus the durability can be further improved.

Here the present invention is not limited to the above embodiments, andcan be varied variously. For example, as the type of the EPS, as shownin FIG. 21A, the column assist type (the turning force of the motor isreduced via the reduction gear to power/energize the column shaft) maybe employed and also, as shown in FIG. 22A, the pinion assist type (theturning force of the motor is reduced via the reduction gear topower/energize the pinion shaft) may be employed.

More particularly, FIG. 21A is a longitudinal sectional view of a columnassist electric power steering according to the present invention, andFIG. 21B is a sectional view showing a pertinent portion of a worm gearsystem of the electric power steering.

In the column assist electric power steering shown in FIG. 21A, a lowercolumn 302 is fitted into an upper column 301 of the steering column onthe front side of the vehicle. An upper shaft 303 and a lower shaft 304(input shaft), which are spline-coupled, of the steering shaft aresupported rotatably in these columns 301, 302.

The output shaft 205 is coupled to the lower shaft 304 (input shaft) onthe front side of the vehicle. The steering gear (not shown) is coupledto this output shaft 205 on the front side of the vehicle via theuniversal joint (not shown), or the like.

The base end of the torsion bar 205 a is press-fitted and fixed to thelower shaft 304 (input shaft) on the front side of the vehicle. Thetorsion bar 205 a is extended through a hollow inner side of the outputshaft 205, and its top end is fixed to an end portion of the outputshaft 5 by a fixing pin 312.

A groove 313 for the torque sensor is formed in the output shaft 205 onthe rear side of the vehicle, and a sleeve 314 of the torque sensor isarranged on the outside of the groove 313 in the diameter direction. Theside end portion of this sleeve 314 on the rear side of the vehicle isfixed to the side end portion of the lower shaft 304 (input shaft) onthe front side of the vehicle by the caulking, or the like. A coil 315,a substrate, etc. are provided on the outside of the sleeve 314 in thediameter direction.

The worm wheel 203 that is meshed with the hourglass worm 202 as thedriving shaft of the electric motor 204 is fitted to the output shaft205.

The steering force generated when the driver turns the steering wheel(not shown) can be transmitted to the turning wheel (not shown) of thevehicle via the input shaft 304, the torsion bar 205 a, the output shaft205, and the rack-and-pinion steering equipment. Also, the turning forceof the electric motor 204 is transmitted to the output shaft 205 via thehourglass worm 202 and the worm wheel 203. Thus, the assist steeringtorque can be applied to the output shaft 205 by controllingappropriately the turning force and the turning direction of theelectric motor 204.

Then, FIG. 22A is a front view containing a partially sectioned crosssection of a pinion assist electric power steering according to thepresent invention, and FIG. 22B is a sectional view showing a pertinentportion of the electric power steering.

In the pinion assist electric power steering, the output shaft 205(pinion shaft) is coupled to a lower shaft 401 (input shaft) on thefront side of the vehicle. A rack 402 of the steering gear is meshedwith the output shaft 205 (pinion shaft). The rack 402 is energizedelastically by an elastic body 403, or the like and is pushed againstthe output shaft 205 (pinion shaft) all the time.

A base end of the torsion bar 205 a is press-fitted and fixed to theoutput shaft 205. The torsion bar 205 a is extended through a hollowinner side of the input shaft 401, and its top end is fixed to an endportion of the input shaft 401.

A groove 404 for a torque sensor is formed in the input shaft 401 on thefront side of the vehicle, and a sleeve 405 of the torque sensor isarranged on the outside of the groove 404 in the diameter direction. Acoil 406, a substrate, etc. are provided on the outside of the sleeve405 in the diameter direction.

The worm wheel 203 that is meshed with the hourglass worm 202 as thedriving shaft of the electric motor 204 is fitted to the output shaft205.

Therefore, the steering force generated when the driver turns thesteering wheel (not shown) can be transmitted to the turning wheel (notshown) of the vehicle via the input shaft 401, the torsion bar 205 a,the output shaft 205, the rack-and-pinion steering equipment, a tie-rod406, and the like. Also, the turning force of the electric motor 204 istransmitted to the output shaft 205 via the hourglass worm 202 and theworm wheel 203. Thus, the proper assist steering torque can be appliedto the output shaft 205 by controlling appropriately the turning forceand the turning direction of the electric motor 204.

Also, as the type of the electric motor 204, either a DC brush motor ora brushless motor may be employed.

The brushless motor can achieve higher the effect of the presentinvention than the brush motor.

In detail, because a brush resistance loss in the brush motor is notgenerated in the brushless motor, the brushless motor can get a betterefficiency and a lower internal resistance, and thus an efficiency ofthe brushless motor is improved further as a high speed motor. In thiscase, a sliding speed of the worm to the worm wheel 203 is increased asthe number of revolution of the worm 202 (220) in the reduction gear isincreased. Therefore, when the brushless motor is used as the electricmotor in the reduction gear, a reduction of the durability due to a lackof the grease becomes more conspicuous. As a result, the effect of thepresent invention can be further enhanced.

Also, in the first and fifteenth embodiments, the number of threads ofthe worm 202 (220) is set forth as two threads. But the effect of thepresent invention is not varied when three threads or one thread isemployed.

Sixteenth Embodiment

FIG. 23 is a longitudinal sectional view of an electric power steeringaccording to a sixteenth embodiment of the present invention.

FIG. 24A is a longitudinal sectional view of the electric power steeringshown in FIG. 23, and FIG. 24B is a schematic view showing arelationship between the pitch circle of the hourglass worm and thepitch circle of the wheel.

FIG. 25A is a longitudinal sectional view of the electric power steeringshown in FIG. 23, FIG. 25B is a schematic view showing a relationshipbetween a pitch circle envelope of the hourglass worm and the pitchcircle of the wheel, and FIG. 25C is a schematic view showing an extentof the backlash.

As shown in FIG. 23, in the sixteenth embodiment, a hourglass worm 502and a worm wheel 503 that is meshed with the hourglass worm 502 areinstalled into a gear housing 501, and an electric motor 504 for drivingthe hourglass worm 502 is mounted on the side portion of the gearhousing 501. The worm wheel 503 is fitted onto an output shaft 505(e.g., a pinion shaft or a column shaft) of the steering equipment.Accordingly, the assist steering torque is generated from the electricmotor 504 in response to the steering torque applied to the steeringwheel (not shown), and then transmitted the output shaft 505 of thesteering equipment via the reduction gear that consists of the hourglassworm 502 and the wheel 503. Here, the reference numeral 505 a denotes atorsion bar.

In the case of the hourglass worm 502, the same fitting way as thecylindrical worm cannot be applied owing to the interference of thepitch circles. Therefore, bearings 506, 507 are fitted from both endsides in the situation that the hourglass worm 502 is meshed with thewheel 503. In other words, the bearings 506, 507 for bearing rotatablyboth end portions of the hourglass worm 502 are fitted such that theirpositions can be adjusted by a snap ring 508 (motor fitting hole 510side) and a cover 509 (shaft end side) respectively. The end surfacepositions of the bearings 506, 507 are adjusted by adjusting the endsurface positions of the snap ring 508 and the cover 509, or the like,and thus the misalignment can be corrected.

By the way, as shown in FIG. 24B, in the case of the hourglass worm 502,a distance between the rotating shaft of the wheel 503 and the rotatingshaft of the hourglass worm 502, both shafts being neither parallel norintersecting, is increased from the shortest distance, which is given asa length of the common perpendicular line to both rotation shafts(center-to-center distance) in accordance with a rotation phase of thewheel 503.

Then, an increment a of the pitch circle radius of the hourglass worm502 is given byσ=R−√{square root over (R ² −X ² )}  [Formula 2]where R is a pitch circle radius of the wheel 503, and

-   -   X is a distance from the common perpendicular line of the        hourglass worm 502.

Therefore, the pitch circle diameter of the hourglass worm 502 issymmetrically increased continuously as a position X goes away from theposition (X=0), on which the common perpendicular line is located and atwhich the minimum diameter is given, in the axial direction of the worm.

In contrast, suppose that an increment of the pitch circle radius of thehourglass worm 502 is σ1, a curvature of a pitch circle radius envelopeof the hourglass worm 502 is R1, and a pitch circle radius of the wheel503 is R (R1>R),σ1=R1−√{square root over (R1 ² X ² )}<σ  [Formula 3]is satisfied. Here, R1 may be set as a constant and σ 1 may be set as afunction that is increased in response to an increase of any X.

The interference between the pitch circle of the wheel 503 and the pitchcircle of the hourglass worm 502 due to a positional displacement of thehourglass worm 502 is very small in the center portion of the hourglassworm 502 but becomes large on both end sides.

As shown in FIG. 25B, if a curvature of an envelope obtained byconnecting the pitch circle of the hourglass worm 502 is set larger thana pitch circle radius of the wheel 502, the minimum backlash of thehourglass worm 502 is not increased but the backlash on both end sidesof the hourglass worm 502 can be increased, as shown in FIG. 25C.

The interference between the tooth surfaces due to the misalignment canbe prevented not to enlarge the touching sound of the tooth surface dueto the backlash, and also a tolerance in the adjusting operation can berelaxed. Therefore, the productivity can be improved.

Also, the wheel 503 is formed to be flexible if at least the toothportion of the wheel is made of a synthetic resin. Thus, the number ofworking teeth between the wheel 503 and the hourglass worm 502 can beincreased sequentially in response to the transmission torque.

As a consequence, an increase of a contact pressure that is increased inanswer to an increase of the transmission torque can be suppressed smallby expanding a loading range, and also the abrasion durability can beimproved.

In addition, if the number of threads of the hourglass worm 502 isincreased, the number of working teeth at full load is increased.Therefore, an expansion of the loading range can be attained smoothly inresponse to the transmission torque and also an increase of the facepressure can be made smoother, and thus the abrasion durability can beimproved.

Also, in the case of the hourglass worm 502, the hourglass worm 502 isnot fitted in the same way as the cylindrical worm because the pitchcircles interfere with each other. Thus, the bearings are fitted fromboth end sides in the situation that the hourglass worm 502 is meshedwith the wheel 503. The misalignment is corrected by adjusting the endsurface position of the bearing 506 by using the snap ring 508, or thelike.

Also, in the reduction gear using the above EPS hourglass worm, aninfluence of an assembling error of the hourglass worm in the axialdirection is exerted largely rather than the reduction gear using thecylindrical worm. In the cylindrical worm, the engagement is not varieddepending on the worm position in the axial direction. In contrast, sucha problem is raised that, if an assembling error of the hourglass wormin the axial direction becomes large in the hourglass worm, a margin ofthe engagement between the worm and the worm wheel is lost completelyand then the frictional resistance that weakens the driving force isgenerated, i.e., the meshing confliction is generated in some portion.

For example, it is apparent from FIG. 42B that no meshing confliction isgenerated in the engagement between the hourglass worm 502 and the wormwheel 503 shown in FIG. 42A. Here, assume that the right-hand side froma center of the shaft of the worm wheel 503 in FIG. 43A is set as the(+) direction and the left-hand side in FIG. 43A is set as the (−)direction. Then, as shown in FIG. 43A, if a fitting error d is generateddue to a deviation of the fitting position of the hourglass worm 502along the (+) direction of the axial direction, the meshing conflictionis generated in the engagement between the hourglass worm 502 and theworm wheel 503 (portion indicated by P), as shown in FIG. 43B. Suchinfluence is enhanced toward the (−) direction from the center of theworm 502.

Similarly, as shown in FIG. 44A, if a fitting error d is generated dueto a deviation of the fitting position of the hourglass worm 502 alongthe (−) direction of the axial direction, the meshing confliction isgenerated in the engagement (portion indicated by P), as shown in FIG.43B. Such influence is enhanced toward the (+) direction from the centerof the worm 502.

In this manner, when the meshing confliction is generated between thehourglass worm 502 and the worm wheel 503, such meshing conflictioncauses a defective operation, a reduction of the efficiency, etc. of thereduction gear, which leads to a reduction of the working efficiency ofthe EPS. As a result, such an disadvantage is caused that theturning-back of the steering wheel is worsened.

For this reason, in embodiments of the present invention described inthe following, the electric power steering that is capable ofsuppressing the influences of the meshing confliction, and the likecaused by the assembling error of the reduction gear using the hourglassworm in the worm axial direction to the utmost will be provided.

Seventeenth Embodiment

FIG. 26A is a longitudinal sectional view showing an hourglass wormreduction gear in an electric power steering according to a seventeenthembodiment of the present invention, and FIG. 26B is an enlarged view ofan engaging portion. FIG. 27 is an enlarged view showing an hourglassworm in FIG. 26A.

As shown in FIG. 26A, in the seventeenth embodiment, the hourglass worm502 and the worm wheel 503 that is meshed with the hourglass worm 502are installed into the gear housing 501 of the worm gear system, and theelectric motor 504 for driving the hourglass worm 502 is mounted on theside portion of the gear housing 501. The hourglass worm 502 is fittedrotatably in the gear housing 501 via the bearings 506, 507 fixed in thegear housing S01. The worm wheel 503 is fitted/fixed onto the outputshaft 505 (e.g., the pinion shaft or the column shaft) of the steeringequipment. The torsion bar 505 a is fitted into the output shaft 505.

According to this configuration, the assist steering torque obtained bydecelerating the driving force of the electric motor 504 via thehourglass worm 502 and the wheel 503 is generated in response to thesteering torque applied to the steering wheel (not shown) and thentransmitted the output shaft 505 of the steering equipment.

The hourglass worm 502 cannot be fitted in the same way as thecylindrical worm because the pitch circles interfere with each other.Therefore, the bearings 506, 507 are fitted from both end sides in thesituation that the hourglass worm 502 is meshed with the worm wheel 503.In other words, the bearings 506, 507 are fitted such that theirpositions can be adjusted by the snap ring 508 (motor fitting hole 510side) and the cover 509 (shaft end side) respectively. The end surfacepositions of the bearings 506, 507 are adjusted by adjusting the endsurface positions of the snap ring 508 and the cover 509, or the like,and thus the misalignment can be corrected.

As shown in FIG. 27, the tooth thickness adjusting process is applied toa profile indicated by a broken line to reduce/thin each tooth thicknessby an infinitesimal amount, and thus the hourglass worm 502 is shapedinto a profile indicated by a solid line.

FIG. 26A shows the condition that the worm wheel 503 is turned CCW(counter-clockwise) by the input given by the direct action of the worm502. In this condition, as shown in FIG. 26B, no meshing confliction isgenerated as a whole in the engagement between the hourglass worm 502and the worm wheel 503.

FIG. 27A is a longitudinal sectional view showing the reduction gearthat contains the assembling error of the hourglass worm, to which thetooth thickness adjusting process is applied, in the axial direction (+direction) and FIG. 27B is an enlarged view of an engaging portion. FIG.28A is a longitudinal sectional view showing the reduction gear thatcontains the assembling error of the hourglass worm, to which the tooththickness adjusting process is applied, in the axial direction (−direction) and FIG. 28B is an enlarged view of an engaging portion.

In the above configuration, assume that the right-hand side from thecenter of the shaft of the worm wheel 503 in FIG. 28A is set as the (+)direction and the left-hand side in FIG. 28A is set as the (−)direction, for example, when the assembling error is generated in theaxial direction of the hourglass worm 502 upon assembling the reductiongear (worm gear system) Then, as shown in FIG. 28A, even though thefitting error is generated to deviate the fitting position of thehourglass worm 502 by a distance d along the (+) direction of the axialdirection, the meshing confliction the influence of which is enhancedlarger toward the (−) direction from the center of the worm 502 in theexample in FIG. 43B can be suppressed to the lowest minimum in theengagement between the hourglass worm 502 and the worm wheel 503, asshown in FIG. 28B.

Similarly, as shown in FIG. 29A, even though the fitting error d isgenerated to deviate the fitting position of the hourglass worm 502along the (−) direction of the axial direction, the meshing conflictionthe influence of which is enhanced larger toward the (+) direction fromthe center of the worm 502 in the example in FIG. 44B can be suppressedto the lowest minimum in the engagement, as shown in FIG. 29B.

As a consequence, even though the assembling error of the hourglass worm502 in the axial direction is generated upon assembling the reductiongear, generation of the meshing confliction in the engagement betweenthe hourglass worm 502 and the worm wheel 503 can be suppressed as smallas possible since the tooth thickness adjusting process is applied tothe hourglass worm 502. Also, a defective operation, a reduction of theefficiency, etc. of the reduction gear can be suppressed.

Eighteenth Embodiment

Next, an eighteenth embodiment of the present invention will beexplained with reference to FIG. 30A to FIG. 34B hereunder.

FIG. 30A is a longitudinal sectional view showing an hourglass wormreduction gear in an electric power steering according to an eighteenthembodiment of the present invention, and FIG. 30B is an enlarged view ofan engaging portion. FIG. 31 is an enlarged view showing an hourglassworm in FIG. 30A.

The eighteenth embodiment is substantially similar to the aboveseventeenth embodiment, and the same reference numerals are affixed tothe same members and parts as the above embodiment and thus theirredundant explanation will be omitted herein. A difference between themresides in that, as shown in FIG. 30A and FIG. 31, the tooth thicknessof the hourglass worm 502 is thinned gradually from the center to bothend portions in the axial direction in the tooth thickness adjustingprocess. As shown in FIG. 31, the tooth profile of the hourglass worm502 is shaped from a shape indicated by a broken line to a shapeindicated by a solid line by the tooth thickness adjusting process. InFIG. 31, a central portion of the hourglass worm 502 is seldom processedor processed only by a minute amount, and an amount of cut of the toothprofile is increased toward both end portions.

FIG. 30A shows the condition that the worm wheel 503 is turned CCW(counter-clockwise) by the input given by the direct action of thehourglass worm 502. In this condition, as shown in FIG. 30B, no meshingconfliction is generated in the overall engagement between the hourglassworm 502 and the worm wheel 503.

FIG. 32A is a graph showing a relationship between a tooth thickness ofthe worm and an angle from the center of the worm wheel, and FIG. 32B isa view explaining the graph in FIG. 32A.

As shown in FIG. 32B, if a straight line passing through a center of theworm wheel 503 and a center of the hourglass worm 502 in the axialdirection is defined as L, each position of the hourglass worm 502 fromthe straight line L as the center in right and left directions in FIG.32B is represented by an angle θ that is formed between another straightline M, which passes through this position and the center of the wormwheel 503, and the straight line L. In this case, the graph in FIG. 32Aindicates that the tooth thickness of the hourglass worm 502 is reducedgradually as |θ| is increased, i.e., the position goes closer to bothend portions. In FIG. 32A, a broken line indicates the type that thetooth thickness is reduced gradually as |θ| is increased, a chaindouble-dashed line indicates the type that an extent of reduction of thetooth thickness is increased as |θ| is increased, and a solid lineindicates the type that the tooth thickness is reduced proportionally as|θ| is increased.

FIG. 33A is a longitudinal sectional view showing the reduction gearthat contains an assembling error of the hourglass worm, to which thetooth thickness adjusting process is applied, in the axial direction (+direction), and FIG. 33B is an enlarged view of an engaging portion.FIG. 34A is a longitudinal sectional view showing the reduction gearthat contains the assembling error of the hourglass worm, to which thetooth thickness adjusting process is applied, in the axial direction (−direction), and FIG. 34B is an enlarged view of an engaging portion.

In the above configuration, as shown in FIG. 33A, even though thefitting error is generated to deviate the fitting position of thehourglass worm 502 by a distanced along the (+) direction of the axialdirection, for example, when the assembling error is generated in theaxial direction of the hourglass worm 502 upon assembling the reductiongear, the influence of the meshing confliction appears larger toward the(−) direction from the center of the worm 502 in the example in FIG.42B, nevertheless the meshing confliction can be relaxed/suppressed inthe engagement between the hourglass worm 502 and the worm wheel 503, asshown in FIG. 33B.

Similarly, as shown in FIG. 34A, even though the fitting error d isgenerated to deviate the fitting position of the hourglass worm 502along the (−) direction of the axial direction, the influence of themeshing confliction appears larger toward the (+) direction from thecenter of the worm 502 in the example in FIG. 43B, nevertheless themeshing confliction can be relaxed in the engagement, as shown in FIG.34B.

As a result, like the seventeenth embodiment, a defective operation, areduction of the efficiency, etc. of the reduction gear due to themeshing confliction can be suppressed. Also, an influence of theassembling error in the axial direction is small in the center portionof the hourglass worm 502. Therefore, like the eighteenth embodiment, ifthe tooth thickness processing in the center portion of the hourglassworm 502 is lessened rather than both end portions, an increase of thebacklash caused by the tooth thickness adjusting process can besuppressed.

Also, FIG. 35 is an explanatory view showing an engagement between thehourglass worm, to which the tooth thickness adjusting process isapplied, and the worm wheel at the time of the small torquetransmission. FIG. 36 is an explanatory view showing the engagementbetween the hourglass worm, to which the tooth thickness adjustingprocess is applied, and the worm wheel at the time of the large torquetransmission.

In the small torque transmission of the reduction gear, as shown in FIG.35, the torque can be transmitted via a small extent of engagement suchthat only the teeth in the center portion of the hourglass worm 502 ismeshed with the worm wheel 503. In the large torque transmission of thereduction gear, as shown in FIG. 36, since the worm wheel 503 isdeflected, the torque can be transmitted via a large extent ofengagement such that all teeth in the center portion of the hourglassworm 502 is meshed with the worm wheel 503. In this manner, since thetorque can be transmitted via an extent of engagement that can respondto the torque, such an advantage can be expected that a transmissionefficiency can be improved while maintaining the strength rather thanthe case where the torque is transmitted all the time via a large extentof engagement.

Nineteenth Embodiment

Next, a nineteenth embodiment of the present invention will be explainedwith reference to FIG. 37A to FIG. 41B hereunder.

FIG. 37A is a longitudinal sectional view showing an hourglass wormreduction gear in an electric power steering according to a nineteenthembodiment of the present invention, and FIG. 37B is an enlarged view ofan engaging portion. FIG. 38 is an enlarged view showing an hourglassworm in FIG. 37A.

The nineteenth embodiment is substantially similar to the aboveeighteenth embodiment, and the same reference numerals are affixed tothe same members and parts as the above embodiment and thus theirredundant explanation will be omitted herein. As shown in FIG. 37A andFIG. 38, it is similar to the eighteenth embodiment that the tooththickness of the hourglass worm 502 is thinned gradually from the centerto both end portions in the axial direction in the tooth thicknessadjusting process, but a difference between them resides in that thetooth thickness adjusting process is not applied to a predeterminedinterval in the center portion of the hourglass worm 502. In thehourglass worm 502 shown in FIG. 38, the tooth thickness adjustingprocess is applied to shape the tooth profile from a shape indicated bya broken line to a shape indicated by a solid line. In FIG. 38, theprocess is not applied to an interval W in the center portion of theworm 502, but an amount of process is increased toward both end portionsin remaining intervals to reduce the tooth thickness.

FIG. 37A shows the condition that the worm wheel 503 is turned CCW(counter-clockwise) by the input given by the direct action of thehourglass worm 502. In this condition, as shown in FIG. 37B, no meshingconfliction is generated in the overall engagement between the hourglassworm 502 and the worm wheel 503.

FIG. 39A is a graph showing a relationship between a tooth thickness ofthe worm and an angle from the center of the worm wheel, and FIG. 39B isa view explaining the graph in FIG. 39A.

As shown in FIG. 39B, a position on the hourglass worm 502 with respectto a straight line L, which passes through a center of the worm wheel503 and a center of the worm 502, in right and left directions in FIG.39B is represented by an angle θ that is formed between another straightline M, which passes through this position and the center of the wormwheel 503, and the straight line L. In this case, the graph in FIG. 39Aindicates that the tooth thickness of the hourglass worm 502 is constantbecause the tooth thickness adjusting process is not applied at all toan interval W in the center portion, in which |θ| is within apredetermined range, and the tooth thickness is reduced gradually in arange in which |θ| is increased further. In FIG. 39A, a broken lineindicates the type that the tooth thickness is reduced gradually as |θ|is increased beyond the interval W, a chain double-dashed line indicatesthe type that an extent of reduction of the tooth thickness is increasedas |θ| is increased beyond the interval W, a solid line indicates thetype that the tooth thickness is reduced proportionally as |θ| isincreased beyond the interval W, and a thick broken line indicates thetype that the tooth thickness is reduced linearly and then apredetermined tooth thickness is kept as |θ| is increased beyond theinterval W.

FIG. 40A is a longitudinal sectional view showing a reduction gear thatcontains an assembling error of the hourglass worm, to which the tooththickness adjusting process is applied, in the axial direction (+direction), and FIG. 40B is an enlarged view of an engaging portion.FIG. 41A is a longitudinal sectional view showing a reduction gear thatcontains the assembling error of the hourglass worm, to which the tooththickness adjusting process is applied, in the axial direction (−direction), and FIG. 34B is an enlarged view of an engaging portion.

In the above configuration, as shown in FIG. 40A, even though thefitting error is generated to deviate the fitting position of thehourglass worm 502 by a distanced along the (+) direction of the axialdirection, for example, when the assembling error is generated in theaxial direction of the hourglass worm 502 upon assembling the reductiongear, the influence of the meshing confliction appears larger toward the(−) direction from the center of the worm 502 in the example in FIG.43B, nevertheless such meshing confliction can be relaxed and suppressedin the engagement between the hourglass worm 502 and the worm wheel 503,as shown in FIG. 40B.

Similarly, as shown in FIG. 41A, even though the fitting error d isgenerated to deviate the fitting position of the hourglass worm 502along the (−) direction of the axial direction, the influence of themeshing confliction appears larger toward the (+) direction from thecenter of the worm 502 in the example in FIG. 44B, nevertheless themeshing confliction can be relaxed in the engagement, as shown in FIG.41B.

As a result, like the seventeenth embodiment, the defective operation,the reduction of the efficiency, etc. of the reduction gear due to themeshing confliction can be suppressed. Also, since the influence of thefitting error in the axial direction is small in the center portion ofthe worm 502, an increase of backlash caused by the tooth thicknessadjusting process can be suppressed by providing the interval W, towhich the tooth thickness adjusting process is not applied, in thecenter portion of the worm 502, like the nineteenth embodiment.

In the small torque transmission of the reduction gear, as shown in FIG.35, the torque can be transmitted via a small extent of engagement insuch a manner that only the teeth in the interval, in which the tooththickness adjusting process is not applied, of the hourglass worm 502 ismeshed with the worm wheel 503. In the large torque transmission of thereduction gear, as shown in FIG. 36, the torque can be transmitted via alarge extent of engagement in such a manner that all teeth containing inthe interval, in which the tooth thickness adjusting process is applied,of the hourglass worm 502 is meshed with the worm wheel 503 since theworm wheel 503 is deflected. In this manner, since the torque can betransmitted via an extent of engagement that can respond to the appliedtorque, such an advantage can also be expected that a transmissionefficiency can be improved while maintaining the strength.

According to the seventeenth to nineteenth embodiments, generation ofthe meshing confliction in the engagement between the hourglass worm andthe worm wheel can be suppressed by applying the tooth thicknessadjusting process to the hourglass worm. Also, the defective operation,the reduction of efficiency, and the like of the worm gear system can besuppressed.

Also, since the tooth profile is shaped such that the tooth thickness isthinned toward both end portions from the center portion of thehourglass worm in the axial direction, the torque can be transmitted viaa small extent of engagement in the small torque transmission of thereduction gear whereas the torque can be transmitted via a large extentof engagement in the large torque transmission because of the deflectionof the worm wheel. As a result, the transmission efficiency can beimproved while maintaining the strength.

In addition, the configurations in the fourteenth to nineteenthembodiments may be combined with the configurations in the first tothirteenth embodiments. Therefore, the fitting operation of thehourglass worm can be facilitated remarkably and thus the correction ofthe misalignment can be executed easily.

The present invention is explained in detail with reference to theparticular embodiments as above. But it is apparent for the skilledperson in the art that various variations and modifications may beapplied without departing a spirit and a scope of the present invention.

This application was filed based on Japanese Patent Application (PatentApplication No. 2003-181517) filed on Jun. 25, 2003, Japanese PatentApplication (Patent Application No. 2003-181523) filed on Jun. 25, 2003,Japanese Patent Application (Patent Application No. 2003-181529) filedon Jun. 25, 2003, and Japanese Patent Application (Patent ApplicationNo. 2003-392623) filed on Nov. 21, 2003, and the contents thereof areincorporated herein by the reference.

INDUSTRIAL APPLICABILITY

As described above, according to the present invention, the contactratio can be improved by using the hourglass worm to attain the higheroutput, and also the correction of the misalignment can be performedeasily by simplifying remarkably the fitting operation of the hourglassworm.

Also, according to the present invention, a lubricating performance canbe improved by using the tooth profile with a special shape to improveconsiderably the abrasion durability.

In addition, according to the present invention, the contact ratio canbe improved by using the hourglass worm to attain the higher output, andalso the correction of the misalignment can be attained easily bysimplifying remarkably the positioning operation of the hourglass worm.

1. The electric power steering according to claim 14, wherein at leastone of bearings that bear rotatably the hourglass worm is composed of atapered roller bearing, an angular contact bearing, or a magneto ballbearing, from which an outer ring can be separated.
 2. The electricpower steering according to claim 14, wherein a bearing holder isprovided to at least one of bearings that bear rotatably the hourglassworm, and is put onto an outer ring of the bearing and has a tapersurface on an outer peripheral surface, and a taper hole with which thetaper surface of the bearing holder is engaged is formed in a gearhousing.
 3. The electric power steering according to claim 14, wherein abearing holder is provided to at least one of bearings that bearrotatably the hourglass worm, and is fitted into an inner ring of thebearing and has a taper surface on an inner peripheral surface, and ataper surface that is engaged with the taper surface of the bearingholder is formed on a shaft end portion of the hourglass worm.
 4. Theelectric power steering according to claim 14, wherein an innerperipheral surface of an inner ring of at least one of bearings thatbear rotatably the hourglass worm is formed as a taper surface, and ataper surface that is engaged with the taper surface of the inner ringis formed on the hourglass worm.
 5. The electric power steeringaccording to claim 14, wherein at least one of bearings that bearrotatably the worm wheel is a tapered roller bearing, an angular contactbearing, or a magneto ball bearing, from which an outer ring can beseparated.
 6. The electric power steering according to claim 14, whereina bearing holder is provided to at least one of bearings that bearrotatably the worm wheel, and is put onto an outer ring and has a tapersurface on an outer peripheral surface and a taper hole with which thetaper surface of the bearing holder is engaged is formed in a gearhousing.
 7. The electric power steering according to claim 14, whereinat least one of bearings that bear rotatably the hourglass worm isprovided such that a position can be changed with respect to a gearhousing in a center-to-center direction.
 8. An electric power steeringfor generating an assist steering torque from an electric motor inresponse to a steering torque applied to a steering wheel, and thentransmitting the assist steering torque to an output shaft of a steeringequipment after a speed reduction using a worm gear system, wherein theworm gear system causes a worm driven by the electric motor to mesh witha worm wheel provided to the output shaft, and a tooth profile of theworm wheel and a tooth profile of the worm are shaped into a specialtooth profile in which a first line of contact and a second line ofcontact, which intersect with a sliding direction of the worm andintersect with each other, and a tooth surface of an intermediate gearis formed as a conical surface.
 9. An electric power steering accordingto claim 8, wherein at least a dedendum shape of the worm is formed as ahourglass shape.
 10. An electric power steering according to claim 8,wherein a consistency of a grease is set to 385 or less.
 11. An electricpower steering according to claim 10, wherein a width of the worm wheelis formed wider than a minimum dedendum circle diameter of the hourglassworm.
 12. An electric power steering according to claim 10, wherein aclearance on both end sides is set larger than a clearance in a centerportion of the worm wheel in a tooth trace direction.
 13. An electricpower steering according to claim 10, wherein the electric motor is abrushless motor.
 14. An electric power steering for generating an assiststeering torque from an electric motor in response to a steering torqueapplied to a steering wheel, and then transmitting the assist steeringtorque to an output shaft of a steering equipment after a speedreduction using a worm gear system, wherein the worm gear system causesa hourglass worm driven by the electric motor to mesh with a worm wheelprovided to the output shaft.
 15. An electric power steering accordingto claim 14, wherein a backlash on both end portions of the hourglassworm is set larger than a backlash in an engagement center portion ofthe hourglass worm.
 16. An electric power steering according to claim14, wherein a number of working teeth between the hourglass worm and theworm wheel is increased in response to a transmission torque.
 17. Anelectric power steering according to claim 16, wherein at least one ofworking teeth of the hourglass worm and the worm wheel can be deformedelastically.
 18. An electric power steering according to claim 17,wherein at least tooth portion of the worm wheel is formed of resinmaterial.
 19. An electric power steering according to claim 18, whereina number of threads of the hourglass worm is set to 2 threads or more.20. An electric power steering according to claim 14, wherein a tooththickness adjusting process of reducing each tooth thickness is appliedto the hourglass worm.
 21. An electric power steering according to claim20, wherein the tooth thickness adjusting process of the tooth thicknessadjusting process shapes a tooth profile such that a tooth thickness isthinned toward both end portions from a center portion of the worm in anaxial direction.
 22. An electric power steering according to claim 20,wherein the tooth thickness adjusting process of the tooth thicknessadjusting process shapes the tooth profile such that the process is notapplied to a predetermined interval of the center portion of the worm inthe axial direction and the process is applied to remaining intervalseither to reduce the tooth thickness toward both end portions or to forma constant tooth thickness that is thinner than the tooth thickness inan interval in which the process is not applied.